[asan] Move instrumented null-terminated strings to a special section, LLVM part

On Darwin, simple C null-terminated constant strings normally end up in the __TEXT,__cstring section of the resulting Mach-O binary. When instrumented with ASan, these strings are transformed in a way that they cannot be in __cstring (the linker unifies the content of this section and strips extra NUL bytes, which would break instrumentation), and are put into a generic __const section. This breaks some of the tools that we have: Some tools need to scan all C null-terminated strings in Mach-O binaries, and scanning all the contents of __const has a large performance penalty. This patch instead introduces a special section, __asan_cstring which will now hold the instrumented null-terminated strings.

Differential Revision: https://reviews.llvm.org/D25026



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285619 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index d77a0ae31b7..df068e88090 100644
--- a/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -1535,6 +1535,14 @@ bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
     NewGlobal->copyAttributesFrom(G);
     NewGlobal->setAlignment(MinRZ);
 
+    // Move null-terminated C strings to "__asan_cstring" section on Darwin.
+    if (TargetTriple.isOSBinFormatMachO() && !G->hasSection() &&
+        G->isConstant()) {
+      auto Seq = dyn_cast<ConstantDataSequential>(G->getInitializer());
+      if (Seq && Seq->isCString())
+        NewGlobal->setSection("__TEXT,__asan_cstring,regular");
+    }
+
     // Transfer the debug info.  The payload starts at offset zero so we can
     // copy the debug info over as is.
     SmallVector<DIGlobalVariable *, 1> GVs;